Heavy ion collisions reveal the earliest instants of our Universe

LHC and RHIC provide a completely different view of fundamental forces.

The LHC's proton collisions, which have now successfully nailed down the existence of the Higgs boson, get most of the attention, both in the media and at CERN itself. But, for a few weeks each year, the collider is switched over to smashing lead ions. Heavy ion collisions, in fact, are considered to provide such distinct information that the US has kept open the Relativistic Heavy Ion Collider, which is dedicated to smashing heavy ions, even as it shut down the Tevatron, its dedicated proton/antiproton collider.

Right on the heels of the Higgs announcement, Science is running a review of heavy ion collisions, which nicely explains why they tell us something completely different from what's revealed by proton colliders. Plus it provides a nice picture of how the LHC will provide new data, and the upgrades that have taken place at the RHIC to help keep it relevant.

The matter we see around us is comprised mainly of protons and neutrons. These, in turn, are composed of quarks and gluons, which mediate the strong force that binds them together. Because the potency of the strong force increases with distance, breaking up a nucleon (proton or neutron) typically requires high levels of energy that basically blast the nucleon in part. That's precisely the sort of thing that happens during the proton collisions that take place at the LHC.

Heavy ion collisions—lead at the LHC, gold at RHIC—involve huge numbers of nucleons, on the order of 400. That creates a very different environment. The quarks and gluons that spill out of a proton collision tend to have nothing but empty space around them. In a heavy ion collision, the large number of nucleons that are broken apart at once means that, instead of flying into empty space, a given quark or gluon will have the opportunity to interact with those pouring out of nearby nucleons. As a result, for a brief instant, the collisions don't look much like an explosion; instead, it looks more like the boundaries between nucleons melting, leaving behind a sea of quarks and gluons that are interacting.

The resulting material, called a quark-gluon plasma, isn't just interesting on theoretical grounds. In the first moments of the Universe's existence, the energy density was so high that all normal matter was in this state. It took about a second to cool down enough for protons to condense out of the QGP. But, before that second was up, the Universe had gone through its entire inflationary period, sowing the seeds for the large-scale structures we see today.

What actually goes on as the QGP forms? The review divides things up into three stages. In the first portion of things, the newly liberated gluons form a dense mesh of interactions. This sets the stage for phase 2, where the quarks, while under the influence of the fields generated by the gluons, form the actual QGP. This soup of particles quickly "thermalizes," meaning energy becomes evenly distributed among its components. As this happens, the QGP starts to expand.

Despite its extremely high density, the QGP shows a shear viscosity that's tiny, making the QGP one of the closest things we've seen to an ideal quantum liquid. Somewhat surprisingly, its behavior is nicely described by equations used in string theory—to describe a five-dimensional black hole. The correspondence does have limitations, though, and the review suggests that finding ways to extend the comparison might give the experimentalists more things to look for.

In any case, as the QGP expands, its energy density drops, and it eventually reaches the point where it drops below what's necessary to maintain the plasma. At this stage, various particles "freeze out," including nucleons and other more exotic particles.

How do you study this process? One of the simplest ways is to simply track all the particles that freeze out, and trace them back to where they came from. This lets researchers reconstruct the shape of the QGP as particles condense out of it. The collisions also create some highly energetic particles that start outside the plasma, but with a momentum that carries them into it. If these particles are quarks, they can participate in interactions with the QGP. In the case of light quarks, that will slow them down considerably. But heavier quarks will simply radiate off some energy (in the form of gluons) and pass through without losing much momentum, providing a sensitive probe for conditions within the QGP.

This is where the LHC's higher energy could come in handy. The lead ion collisions there are high enough energy that they can produce Z bosons, the carriers of the weak force. These should be able to cross the QGP, and provide a very different probe of the conditions inside it, since it won't interact in the same ways quarks do. So far, the information generated at the LHC has largely extended the findings of RHIC into higher energy domains.

Meanwhile, RHIC isn't standing still. In additions to upgrades to its detectors, the accelerator chain has been modified to provide the ability to collide many different ions. The review's authors, for example, are excited about the prospect of colliding uranium ions. The nucleus of these atoms is asymmetric, so some of the collisions should provide QGPs with distinct shapes, which will vary the transit time that particles take across the plasma. The asymmetric collisions may also provide a glimpse into the earliest steps of forming gluon interaction networks, which remain poorly understood.

In any case, although these collisions aren't going to result in the discovery of new particles, continued work at RHIC and the LHC should provide a clearer picture of the formation and behavior of the quark-gluon plasma and, in the process, give us a better understanding of the Universe's first moments.

Quarks radiate gluons the same way accelerating electrons radiate photons (bremsstrahlung, synchrotron radiation...). It's just a gazillion times more powerful because it's the strong force, not electromagnetism.

bthylafh wrote:

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Because the potency of the strong force increases with distance

Did I misunderstand this sentence, or is it a typo? One of the defining characteristics of the strong force is how short-ranged it is.

It's short ranged because it's so powerful that free particles charged under the strong force are absolutely forbidden, because if you separate two particles charged under the strong force, the force increases until new particles are pulled out of the vacuum to prevent further separation. So, only a pathetic remnant van der Waals force is ever observed, but this force is still powerful enough to be the source of the sun's light and hold nuclei together. Check out asymptotic freedom and confinement.

And this will not provide evidence for string theory. Some of string theory's math is useful, but the states observed are not actually specially predicted by string theory. Notice that it's equations for a 5d black hole that are useful: completely unrelated.

Did I misunderstand this sentence, or is it a typo? One of the defining characteristics of the strong force is how short-ranged it is.

No, it's right when referring to the interaction of particles on the quark/gluon scale, inside a hadron. That's why there are pretty much no free quarks for any significant length of time; they can't escape the strong force. It doesn't drop off with distance. But it only affects gluons and quarks. The residual strong force, the ghostly remnants of the internal actions of strong force that stretches out to link hadrons together into a nucleus, is what diminishes with distance. Inside hadrons the undiminished strong force is mostly canceled out, but not entirely or constantly.

O hell! what have we here?A Cyclotron, within whose empty eyeThere is a written scroll! I'll read the writing.In our splitters is not gold(Lead ions is what we were sold)Heat 'em up so they're not coldThe nascent universe beholdAs Mr Higgs has oft foretoldHad we been as wise as boldHis boson we would soon unfoldBefore the poor bloke is too old.

Sorry, Bill.(And sorry it's a bit off topic; I know the article isn't about the Higgs. I did try to get 'Quark Gluon Plasma', '5D black hole' and 'strong nuclear force' into The Merchant of Venice but they just weren't a good fit. Obviously they'd go well in Hamlet - but I don't like that play).

1) Can the strong and weak forces be explained intuitively like gravity and EM? Or is any description hidden in standard model theory?

2) Why does is seem like some refer to 4 fundamental forces and other times you read about EM and Weak forces being unified?

3) Why do physicists demonstrate gravity is weak using the example of a refrigerator magnet countering the earth's gravity on paperclips, yet it seems like it would take a fair amount of effort to get a paperclip into space?

2) Why does is seem like some refer to 4 fundamental forces and other times you read about EM and Weak forces being unified?

It's a question of energy levels. In ordinary everyday conditions all 4 are distinct. The electromagnetic and weak forces combine at ~100 GeV; which has been within the reach of our accelerators for decades. There are various theories to unify the electro-weak and strong forces at higher energy levels (far beyond the LHC), and others that attempt to do the same with gravity as well (eg string theory); but none are universally accepted and every prediction one's made that has been testable so far has resulted in the theory being falsified.

3) Why do physicists demonstrate gravity is weak using the example of a refrigerator magnet countering the earth's gravity on paperclips, yet it seems like it would take a fair amount of effort to get a paperclip into space?

The inverse square law is at work here. Move the paperclip from 1 to 2cm away from the magnet the force on it drops by a factor of 4. To do the same for gravity you'd need to double your distance from the earth's core. That's roughly 10x as far out as low earth orbit.

1) Can the strong and weak forces be explained intuitively like gravity and EM? Or is any description hidden in standard model theory?

Not very well. The strong force (QCD) has some similarity in that it based on color charge: while E&M has only one type of charge that can be positive or negative, QCD has three: red, green, and blue, each of which can be positive or negative (called anti-red, etc.). In E&M, an object can be neutral if it has equal positive an negative charge, in which case there is no long range force. In QCD, you can have color charges balanced either by having equal amounts of red, green, and blue, or by having equal amounts of a color and its anti-color. However, the details are quite a bit different. The strong force is so strong that it is actually impossible to have a non-neutral particle. Another big difference is that the force carrying particles gluons (equivalent to photons in E&M) carry color charge while photons are electrically neutral. This is part of what makes QCD so hard to calculate -- the particles carrying the force themselves interact by the same force. As you can see by crossing two laser beams, photons don't interact with each other.

The weak force is even harder to explain by analogy, but the important this is that it can do things that the other forces can't, like convert neutrons into protons. Thus, it plays an important role in many nuclear reactions. The relative weakness of it is the reason that we have atomic isotopes that are unstable but with half-lives of millions of years.

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3) Why do physicists demonstrate gravity is weak using the example of a refrigerator magnet countering the earth's gravity on paperclips, yet it seems like it would take a fair amount of effort to get a paperclip into space?

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The important thing about gravity that makes it different from all the other forces is that gravitational "charge" aka mass only comes in positive values. The electromagnetic forces between two fundamental particles is much stronger than gravity, but in practice we always find that larger objects have approximately neutral net charge. Even the force that holds a fridge magnet is actually just a small byproduct of the forces that holds solids together, which is itself a small remnant of the force that holds atoms together. Gravity can't be neutralized and so keeps building up as you add more mass.

"...negative mass will fall toward (and not away from) "normal" matter, since although the gravitational force is repulsive, the negative mass (according to Newton's law, F=ma) responds by accelerating in the opposite of the direction of the force. Normal mass, on the other hand, will fall away from the negative matter..."

TheDS wrote:

"Gravity can't be neutralized and so keeps building up as you add more mass."

I love this science and am always fascinated by what humans can discover, but we (read: journalists) really need to stop using language that makes our theories and speculative behavior sound like we actually 'know' about this stuff. I am not saying that we shouldn't ask or search, but even the title of the article - "Heavy ion collisions reveal the earliest instants of our Universe" - is in reality quite misleading. Makes for good 'news' but no answer has been 'revealed'; on the contrary, it simply opens more questions. While I cannot blame Ars for wanting to put the truly unfathomable into chewable chunks for us plebeians to swallow, it is fundamentally misleading (unprofessional?) to assign that title to this story.

Though we cannot access the linked article without a subscription to that journal, a snippet of the abstract is instructive:

"A description based on string theory and black holes in five dimensions has made the quark-gluon plasma an archetypical strongly coupled quantum system. Open questions about the structure and theory of the quark-gluon plasma are under active investigation. Many of the insights are also relevant to ultracold fermionic atoms and strongly correlated condensed matter."

Nothing there sounds like a 'reveal'. As I said, fascinating stuff, but let's not obfuscate reality for readership.

You can get a good sense for the relative strengths of the forces by looking at particles that interact only through that force or weaker forces.

The strong force is so strong that particles that feel it cannot be separated from each other. Quarks are confined to hadrons, particles with zero net charge under the strong force such as protons, while particles that don't feel the strong force, like electrons, are free to move about.

Electrons feel the electromagnetic force, and are mostly bound in atoms but can be broken off if you try hard enough.

Neutrinos do not feel electromagnetism or the strong force, and can travel through a light year of lead without interacting. Tens of trillions of neutrinos from the sun pass through your body every second, from above at noon and from below after traveling through the Earth at midnight, and yet you will interact with around 1 over the course of your lifetime, if you live to 80 at the equator. The weak force is indeed weak.

Gravity is so weak that the electric force between the electron and proton in a hydrogen atom is roughly 2*10^39 times stronger than the gravitational force. If dark matter interacts only through gravity, which is a completely reasonable hypothesis, humans will never produce it. Perhaps their barely recognizable descendants, but not humans.

You can get a good sense for the relative strengths of the forces by looking at particles that interact only through that force or weaker forces.

The strong force is so strong that particles that feel it cannot be separated from each other. Quarks are confined to hadrons, particles with zero net charge under the strong force such as protons, while particles that don't feel the strong force, like electrons, are free to move about.

Electrons feel the electromagnetic force, and are mostly bound in atoms but can be broken off if you try hard enough.

Neutrinos do not feel electromagnetism or the strong force, and can travel through a light year of lead without interacting. Tens of trillions of neutrinos from the sun pass through your body every second, from above at noon and from below after traveling through the Earth at midnight, and yet you will interact with around 1 over the course of your lifetime, if you live to 80 at the equator. The weak force is indeed weak.

Gravity is so weak that the electric force between the electron and proton in a hydrogen atom is roughly 2*10^39 times stronger than the gravitational force. If dark matter interacts only through gravity, which is a completely reasonable hypothesis, humans will never produce it. Perhaps their barely recognizable descendants, but not humans.

I have been reading about this stuff for years (I am a linguist, not a scientist) but until now, failed to grasp the idea of strong and weak forces with clarity. Thanks for the concise and clear explanation.

<snip>I have been reading about this stuff for years (I am a linguist, not a scientist) but until now, failed to grasp the idea of strong and weak forces with clarity. Thanks for the concise and clear explanation.

I'm assuming the white center indicates a inner space where the detector has no data. So each of the three "brushes" is a separate collision event seen by the detector? The top brush being the most recent?

I love this science and am always fascinated by what humans can discover, but we (read: journalists) really need to stop using language that makes our theories and speculative behavior sound like we actually 'know' about this stuff. I am not saying that we shouldn't ask or search, but even the title of the article - "Heavy ion collisions reveal the earliest instants of our Universe" - is in reality quite misleading. Makes for good 'news' but no answer has been 'revealed'; on the contrary, it simply opens more questions. While I cannot blame Ars for wanting to put the truly unfathomable into chewable chunks for us plebeians to swallow, it is fundamentally misleading (unprofessional?) to assign that title to this story.

The QGP had never been produced prior to RHIC. The definitive paper from RHIC that confirmed what it was seeing was QGP is less than five years old (although non=definitive evidence was discussed earlier than that). Given that, prior to the advent of these heavy ion colliders, the QGP was a theoretical construct, and we had no empirical evidence of its existence or behavior (people definitely weren't expecting a perfect quantum fluid), i don't think there's anything misleading about using the term "reveal" in the title.

I'm assuming the white center indicates a inner space where the detector has no data. So each of the three "brushes" is a separate collision event seen by the detector? The top brush being the most recent?

No, those aren't collision events - they're the individual particles that have frozen out of the QGP as it cooled.

Yep, each bristle is one particle. Most of them are probably protons, antiprotons, neutrons, etc., but you get some heavier stuff, like the equivalents carrying a strange or charm quark. In some cases, some of these particles end up so close together, the condense into even heavier things - i think RHIC has popped out a couple of anti-helium nuclei.

2) Why does is seem like some refer to 4 fundamental forces and other times you read about EM and Weak forces being unified?

That is, arguably, dated as of 2 weeks ago. Most generically, force causes a change in an object. Now we have evidence for a 5th fundamental force* that gives mass to many Standard Model particles as the universe cools down, the higgs mechanism. We may not be used to think of universal scalar fields as forces (or at all), making an early change as they do, but likely we should. Then SM have 3 forces (EM, weak, strong), SM+higgs means 4, gravity makes 5.

EM and weak forces are explicitly unified in electroweak theory, and we know and can probe when it happens. The Standard Model joins the strong force to this, but is isn't as explicit and fully known to merit "unification" as of yet.

----------* Dark energy changes the universe from freewheeling to accelerated expansion, but it is an open question whether it is fundamental. If it is a cosmological constant it could be residual vacuum energy, which in turn is composed of the fields of forces and particles.

WhitneyLand wrote:

3) Why do physicists demonstrate gravity is weak using the example of a refrigerator magnet countering the earth's gravity on paperclips, yet it seems like it would take a fair amount of effort to get a paperclip into space?

You are comparing apples with pears. Gravity is the weakest force per unit charge compared at unit distance. If you scale up a refrigerator magnet to the size of Earth, you will find it very hard to launch iron rockets.

Note though that this is still apples and pears, since the monopole field of gravity (one type if charge) weakens as r^-2, while the dipole field of a magnet similar to an electrostatic dipole (two types of charges) weakens as r^-3.

Remember that while mass is a mess (many mechanisms), forces are fucked up (many differences).

The rhic has already collided Uranium Ions . This past run did it for the first time. It was more of a test but they are now working on studying the data.

Also the RHIC can also control proton spin which the lhc cannot do. They do not know what makes protons spin. They theorize is could be a new particle or something else. That's another reason why the rhic is staying open . Brookhaven also states they are using the different energies of the RHIC, LHC and another heavy ion accelerator with less energy then the rhic in Europe to work on the nuclear phase diagram.

If you live in the NYC area and like particle physics and the LHC and RHiC on August 5 they have the summer sunday for the RHIC. ITs completely free and you get to tour the rhic and talk to the actual scientists working at the rhic . some of them have even worked at the lhc also.

Will ArsTechnica be going over what was presented at quark Matter 2012 when it happens? http://qm2012.bnl.gov/

That said, the headline is a little hyperbolic. The collision of heavy ions shows conditions that MAY resemble the earliest moments of our universe, but what it does resemble, a lot in fact, is the collision of heavy ions.

That said, the headline is a little hyperbolic. The collision of heavy ions shows conditions that MAY resemble the earliest moments of our universe, but what it does resemble, a lot in fact, is the collision of heavy ions.

2) Why does is seem like some refer to 4 fundamental forces and other times you read about EM and Weak forces being unified?

The first time I heard that 3 of the 4 fundamental forces have been unified was about 20+ years ago when I was still a kid watching some science show about a unified equation. The unification refers to the mathematical formulation or origin equation, and not their being distinct forces or not. Unification basictly relies on math with physical interpretations. There are still areas of math we have not been able to find connections to regular math.